Field of the Invention
The present invention relates to a submarine flexible pipe.
Description of the Related Art
Submarine oil extraction requires the use of flexible pipes for transporting crude oil, along with other substances or gases mixed thereto, from the submarine extraction well to the sea level. Such pipes must be able to withstand for a very long time (at least 20 years) to high internal and/or external pressures (even higher than 1000 bar) and to high temperatures (even up to 130° C.) as well as to high temperature fluctuations correlated to the very high depth below the sea level (even higher than 1000 m). These flexible pipes must also be able to withstand to the continuous bending stresses caused by the marine streams and surface weaves.
In order to cope with the above requirements, submarine flexible pipes provided with a flexible steel structure are widely used. These flexible pipes typically comprise a flexible internal stainless steel carcass, an internal polymeric sheath surrounding the carcass aimed at ensuring the fluid tightness, a helical metal reinforcement layer (pressure armor layer), wound with a short pitch, surrounding the polymeric sheath and adapted to withstand to the internal and/or external pressure, tensile armor layers surrounding the pressure armor layer and adapted to withstand to the longitudinal tensile forces and an external protective polymeric sheath. The tensile armor layers are typically made of carbon steel wires or strips helically wound with a long pitch in opposite directions, possibly separated by polymeric layers.
WO 2008119676 discloses a flexible pipe suitable for transporting crude oil, including hydrocarbons liquid and gases, such as natural gas, in deep offshore oil exploration of depths up to 3000 m. The flexible pipe comprises, from the inside to the outside, a flexible metal tube or carcass, an internal sealing sheath or pressure sheath placed around the carcass, a covering plastic layer between the carcass and the pressure sheath, a pressure vault intended to take up the internal compressive forces, two tensile armor plies intended to take up the longitudinal tensile forces and a protective sheath. The tensile armor plies are formed of metal wires or strips wound with a long pitch.
During the whole working time of the flexible pipe, some gases mixed with the crude oil, mainly methane, carbon dioxide, hydrogen sulfide and water, slowly diffuse through the internal polymeric sheath and reach the tensile armor layers. Carbon dioxide, hydrogen sulfide and water can give rise over the time to corrosion phenomena of the carbon steel and, in particular when a high resistance carbon steel (e.g. Rockwell Hardness Cone>20 kg/mm2) is employed, can give rise to an undesired steel embrittlement, which considerably limits the working life of the flexible pipe.
The Applicant observed that a flexible pipe as disclosed in WO 2008119676 is subjected to the above-discussed corrosion phenomena, which considerably limits the pipe working life.
U.S. Pat. No. 4,773,151 discloses a flexible hose suitable for the transportation of crude oil containing hydrogen sulfide. The hose comprises a body of elastomeric material having embedded therein two wire reinforcement layers. Each reinforcement layer comprises twenty cords each of an assembly of seven wire filaments, said cords being arranged to extend helically relative to the direction of the length of the hose. Each wire filament has a steel core surrounded by a coating of aluminum for the purpose of providing the steel with improved corrosion resistance to the hydrogen sulfide.
The Applicant observed that the aluminum coated steel wires disclosed in U.S. Pat. No. 4,773,151 are fully insulated, being embedded in an elastomeric material.
However, the steel armors in a submarine flexible pipe are not insulated, being located in a position where the contact with the seawater has to be contemplated.
As a matter of fact, the location of the steel armors under the external sheath is a place exposed to seawater contact, for example because of damages occurring to the external sheath during the flexible pipe operating life and possible condensation of the seawater, with the result of corrosive problems due to the potential presence of water, carbon dioxide and hydrogen sulfide.
This is accounted, for example, in the Conference Paper “Qualification of Steel Wire for Flexible Pipes” by Adam Rubin and Jonas Gudme, NKT Flexibles, published by NACE International in CORROSION 2006, Mar. 12-16, 2006, San Diego Calif.
In particular, such paper refers to the space between the inner liner and the outer sheath, referred to as the “annulus”. This annular space is primarily occupied with carbon steel from the armoring wires.
Such paper also defines that the free volume in the annulus between the steel wires is very limited resulting in a very high ratio of steel surface to free volume.
In order to determine the life of flexible pipes, prediction of the annulus environment is of great importance. In principle the annulus environment is determined by the following factors:                Transportation of gas in and out of the annulus through the polymer liners;        Presence of water in annulus and possible condensation;        Corrosion reaction;        Venting valve opening pressure.        
It is indeed well known that the presence of dissolved oxygen and chloride ions reduces the resistance of aluminum to corrosion in water. It is thus not recommendable using aluminum in those applications where it is exposed to a potential contact with the seawater.
This is confirmed by several publications. For example, “The long Island Sound Submarine cable Interconnection Operating Experience” of M. Chamberlin and S. W. Margolin, 7th IEEEPES conference, Apr. 1-6, 1979, Book CH1139-759, pages 290-298, discloses the operating experience of a submarine fluid oil cable for energy transport. The cable comprises an oil duct, a copper conductor, a carbon-black screen, a paper insulation, a lead-alloy sheet, a polyethylene sheath and an aluminum alloy wire armor layer. A 19 km length of 138 KV ac cable was installed at the Long Island Sound. This cable showed corrosion problems, in part due to an incorrect cathodic protection design, in part due to free and galvanic corrosion phenomena caused by the high presence of oxygen in the seawater, even at the sea bottom. This experience has shown the importance of providing physical protection to the cables at any depth in seawater and to be especially cautious in the analysis and design of the cathodic protection system. In its eight years of operation, the cable has experienced 11 incidents of physical damage and/or sheath corrosion. The events of these eight years of operation lead to several judgmental conclusions, applicable to future installations, with similar water and bottom conditions, one of these judgmental conclusions being that the use of an aluminum alloy armor in seawater is not recommended. The submarine cable field is closely related to the flexible pipes field for these aspects, as similar problems are faced by both technologies.